A complete registry will be kept by the central laboratory. All patients will be entered in the study by filing an on-study card and a summary sheet. Separate summary sheets have been designed for all patients free of inhibitors and for patients in whom inhibitors are present. Any non-inhibitor patients subsequently developing an inhibitor will have a notification form forwarded immediately to the central laboratory and will be expected to have an inhibitor patient summary sheet completed within one month. Flow sheets will be used on which patient data will be recorded. Twice yearly, flow sheets will be called up by the central laboratory for data collation. Blood samples sent to the central laboratory by cooperating centers, whether as "random" samples or as samples requested by the central laboratory, must be accompanied by a laboratory report form.